feat(cogify): force a background color for transparent pixels

packages/cogify/src/cogify/cli/cli.cog.ts

@@ -15,7 +15,7 @@ import { CutlineOptimizer } from '../../cutline.js';
 import { SourceDownloader } from '../../download.js';
 import { HashTransform } from '../../hash.stream.js';
 import { getLogger, logArguments } from '../../log.js';
-import { gdalBuildCog, gdalBuildVrt, gdalBuildVrtWarp } from '../gdal.command.js';
+import { gdalBuildCog, gdalBuildVrt, gdalBuildVrtWarp, gdalCreate } from '../gdal.command.js';
 import { GdalRunner } from '../gdal.runner.js';
 import { Url, UrlArrayJsonFile } from '../parsers.js';
 import { CogifyCreationOptions, CogifyStacItem, getCutline, getSources } from '../stac.js';
@@ -332,10 +332,21 @@ async function createCog(ctx: CogCreationContext): Promise<URL> {
   );
   await new GdalRunner(vrtWarpCommand).run(logger);
 
-  logger?.debug({ tileId }, 'Cog:Create:Tiff');
+  // Create a tiff file covering the whole world in sea blue
+  const gdalCreateCommand = gdalCreate(new URL(`${tileId}-bg.tiff`, ctx.tempFolder), options);
+  await new GdalRunner(gdalCreateCommand).run(logger);
+
+  // Create a vrt layering with the sea blue behind the warp VRT
+  const vrtMergeCommand = gdalBuildVrt(new URL(`${tileId}-merged.vrt`, ctx.tempFolder), [
+    gdalCreateCommand.output,
+    vrtWarpCommand.output,
+  ]);
+  await new GdalRunner(vrtMergeCommand).run(logger);
+
   // Create the COG from the warped vrt
-  const cogCreateCommand = gdalBuildCog(new URL(`${tileId}.tiff`, ctx.tempFolder), vrtWarpCommand.output, options);
+  const cogCreateCommand = gdalBuildCog(new URL(`${tileId}.tiff`, ctx.tempFolder), vrtMergeCommand.output, options);
   await new GdalRunner(cogCreateCommand).run(logger);
+
   return cogCreateCommand.output;
 }
 

packages/cogify/src/cogify/gdal.command.ts

@@ -97,3 +97,29 @@ export function gdalBuildCog(targetTiff: URL, sourceVrt: URL, opt: CogifyCreatio
       .map(String),
   };
 }
+
+export function gdalCreate(targetTiff: URL, opt: CogifyCreationOptions): GdalCommand {
+  const cfg = { ...Presets[opt.preset], ...opt };
+  const tileMatrix = TileMatrixSets.find(cfg.tileMatrix);
+  if (tileMatrix == null) throw new Error('Unable to find tileMatrix: ' + cfg.tileMatrix);
+
+  const bounds = tileMatrix.tileToSourceBounds({ x: 0, y: 0, z: 0 });
+
+  return {
+    command: 'gdal_create',
+    output: targetTiff,
+    args: [
+      ['-of', 'GTiff'],
+      ['-outsize', 20, 20],
+      ['-bands', '4'],
+      ['-burn', '208 231 244 255'], // this is the color
+      ['-a_srs', tileMatrix.projection.toEpsgString()],
+      ['-a_ullr', bounds.x, bounds.bottom, bounds.right, bounds.y],
+      ['-co', 'COMPRESS=LZW'],
+      urlToString(targetTiff),
+    ]
+      .filter((f) => f != null)
+      .flat()
+      .map(String),
+  };
+}

packages/cogify/src/cogify/gdal.runner.ts

@@ -8,7 +8,7 @@ export interface GdalCommand {
   /** Output file location */
   output: URL;
   /** GDAL command to use */
-  command: 'gdalwarp' | 'gdalbuildvrt' | 'gdal_translate';
+  command: 'gdal_create' | 'gdalwarp' | 'gdalbuildvrt' | 'gdal_translate';
   /** GDAL arguments to use */
   args: string[];
 }